1. Field of the Invention
The present invention relates to an X-ray analyzer which is used for, for example, an electron microscope or a fluorescent X-ray analysis apparatus to perform energy discrimination of generated X-rays, thereby determining elemental species of a generation source, and more particularly, to an X-ray analyzer using, as an X-ray detector, a transition edge sensor for converting X-ray energy into thermal energy.
2. Description of the Related Art
As an X-ray analyzer capable of performing X-ray energy discrimination, there are known an energy dispersive spectroscopy (hereinafter referred to as EDS) and a wavelength dispersive spectroscopy (hereinafter referred to as WDS).
The EDS is an X-ray detector of a type that converts energy of an X-ray taken in the detector into an electrical signal in the detector and calculates the energy based on a magnitude of the electrical signal. The WDS is an X-ray detector of a type that produces a monochromatic X-ray from an X-ray by a spectroscope (energy discrimination) and detects the monochromatic X-ray by a proportional counter.
As the EDS, there is known a semiconductor detector such as a silicon lithium (SiLi) detector. When the semiconductor detector is used, energy in a wide range of approximately 0 keV to 20 keV can be detected. However, energy resolution is as narrow as approximately 130 eV, which is one tenth or less of the WDS.
In recent years, attention has been given to superconducting X-ray detectors which are of an energy dispersion type and equal in energy resolution to the WDS. Of the superconducting X-ray detectors, a detector called a transition edge sensor (hereinafter referred to as TES) is a high-sensitive thermometer based on a rapid change in resistance (ΔR to 0.1Ω at ΔT to several mK) of a metal thin film at the time of superconduction-normal conduction transition. The TES is also called a microcalorimeter.
In the TES, a sample is irradiated with a radiation such as a primary X-ray or a primary electron beam from a radiation source. When a fluorescent X-ray or a characteristic X-ray which is generated from the sample is caused to enter the TES, temperature inside the TES changes. Therefore, the temperature is controlled to analyze the sample. Currently, an energy resolution equal to or smaller than 10 eV can be obtained as the energy resolution of the TES in a case of, for example, a characteristic X-ray of 5.9 keV (see K. D. Irwin, “An application of electrothermal feedback for high resolution cryogenic particle detection”, Applied Physics Letters, 66, 1995, p. 1998).
When the TES is attached as an electron generation source to a thermal type (such as tungsten filament type) scanning electron microscope, a characteristic X-ray generated from a sample irradiated with an electron beam is obtained. As a result, it is revealed that characteristic X-rays (Si-Ka and W-Ma, b) which cannot be separated in a semiconductor X-ray detector can be easily separated by the TES (see K. Tanaka, et al., “A microcalorimeter EDS system suitable for low acceleration voltage analysis”, Surface and Interface Analysis, 38, 2006, p. 1646).
The TES is provided in a tip end portion of a rod-shaped member called a cold finger which is attached to a cooling device to bring the detector close to the sample, as in the case of the conventional semiconductor EDS. In the case of the TES using the superconducting material, when a magnetic field equal to geomagnetism is applied as an external magnetic field to the sensor, sensitivity deteriorates because of the influence of the magnetic field. Therefore, a magnetic shield against geomagnetism is conventionally provided for a snout housing the cold finger.
The conventional technologies described above have the following problem.
For example, in the case of the TES described in K. Tanaka, et al., “A microcalorimeter EDS system suitable for low acceleration voltage analysis”, Surface and Interface Analysis, 38, 2006, p. 1646, the thermal type (tungsten filament type) scanning electron microscope and the TES are separated from each other by several centimeters, and there is employed a structure in which a magnetic field is prevented from leaking from a barrel of the electron microscope. Therefore, the influence of the external magnetic field on the sensitivity of the TES is not observed. However, in a case of a high-resolution electron microscope (for example, field emission electron microscope), a fringing field is likely to influence the sensitivity of the TES. That is, in such an electron microscope, an in-lens type or semi-in-lens type objective lens in which the magnetic field is caused to leak outside the barrel is the mainstream, and a strong magnetic field is applied to converge primary electrons emitted from a field emission cathode, and thus the fringing field is likely to influence characteristics of the TES. It is highly conceivable that the TES will be used in a generated magnetic field equal to or larger than geomagnetism, in addition to the cases of the electron microscope and the fluorescent X-ray analysis apparatus. Thus, it is desired to stably operate the TES in a magnetic field equal to or stronger than geomagnetism.